Referring to FIG. 1, a conventional venetian blind 1, as disclosed in Taiwanese Utility Patent No. M306267, includes a frame 11, a plurality of slats 12 rotatably connected to the frame 11, a plurality of rotary members 13 each connected fixedly to an end of a respective one of the slats 12, a bead chain 14 trained on the rotary members 13, and an operating member 15 connected to the bead chain 14 and movable relative to the frame 11. When the operating member 15 is moved upward or downward relative to the frame 11, the operating member 15 drives rotation of the bead chain 14 so as to rotate the rotary members 13. The slats 12 rotate together with the rotary members 13 so that the slat angle is adjusted.
However, each of the slats 12 has an angle of rotation less than 90 degrees, which means that the bead chain 14 has to drive the rotation of the rotary members 13 by moving a relatively short distance, which is relatively difficult to operate and may be laborious for an operator.